X-ray filter



Aprll 12, 1927. A. ST JOHN X-RAY FILTER Filed Oct. 20. 1921 IN V EN TOR.

ATTORNEYS.

Patented Apr. 12, 1927..

UNITED STATES PATENT OFFICE. L

ANGEL ST. JOHN, Oil?" IBROAOIKII'.|YN,l NEW YORK, ASSIGNOR TO UNIONCARBIDE & CARBON RESEARCH LABORATORIES, INC.,

A CORPORATION .OF NEW YORK.

X-RAY FILTER..

y Application mea october 2o,

'Ihis'invention relates to filters adapted for use in connection withX-ray work, especially for the production of lphotographic images byX-rays.

Every substance, when properly' excited, as by the action of cathoderays, emits a spectrum of X-rays of various wave-lengths, and the raysofhone particular Wave-length are considerably more I intense than anyother. In some classes of X-ray work it is desirable to suppress as faras possible all the. wave-lengths of a particular X-radiation with theexception of a single portion of the spectrum, so that a radiationconfined to a limited range of wave-lengths including that of maximumintensity, is obtained. -Such a radiation is analogous to monochromaticlight.

The elimination of all but the most intense radiation may beaccomplished by the interposition of a suitable selectively absorbent orfiltering material in the path of the rays. A material containing thesame chemical 4.element as that from which the X-rays are emitted willtransmit the most intense radiation (usually calledthe Kaline) fairlyWell,- provided the material does not also contain an element whichstrongly absorbs the Kor-line. Such a material will also absorb much ofthe remainder ofthe spectrum, but` it does not mean all the requirementsof a filter because it transmits more of the radiation of second highestintensity (usually called the K-line) than is desirable. A` much betterfiltering action is obtained with a material' in -Which the dominatingelement is'one of slightly lower atomic weight than the element emittingthe rays, since such amaterial absorbs the K- line quite effectually,while transmitting the Kor-line fairly well. The wave-lengths of theX-ray spectrum as a whole increase with decreasing atomic weight .of theemitting element, and the best filtering action is obi" tained when lthefiltering element is so chosen that the shortest wave-length of itsspectrum is intermediate 'the wave-lengths of the Ka' and K lines' ofthe radiation to' be filtered. Thus. when the X-rays emanate from amolybdenum target, materials comprising molybdenum (atomic weight 96)make poor filters. lmaterials' containing columbium (atomicAweight`93.5)l are somewhat better, while materials containing 1921.`serial No. 509,174.

zrcopium (atomic weight 90.6)` are best o al. l

Regardless of .which filtering material is used, the Kor-line isconsiderablyweakened` by absorption in the filter, and the time requiredto secure a photographic image of normal density is increased. It isknown that the time of exposure in X-ray photography can be decreased ifa fluorescent screen is arranged to intercept'4 the rays, transform theenerg contained in them into fluorescent light, and7 project the latterupon :the photographicsurface to add its effect tothat ofthe X-rays.

Such materials as have been used hereto# fore for fiuorescent screensfor the above purpose are not adapted for filters and are best usedbehind the photographic film.

The primary object of my linvention is to provide means for absorbingundesired Wave-lengths :Eroman X-ray spectrum to 'prevent their actionon a photographic sury face. while utilizing actinically the energyofthat portion of the desired `Wavelength which is incidentally absorbedwiththe undesired wave-lengths. I attain this object by usinga'ihaterial which fulfills the necessary requirements fora filter andwhich is fiuorescent` placed in front of the photographic film, it beingunderstood that fluorescentlight will be emitted from the surface towardthe film only at the places where X-rays are emerging from the filter.

ing material, as any light produced by wave- I showl an X-ray tube 1havinga target 2.

lengths which only partially penetrate the particular source of rays.`In the drawing v A lead screen 3 is employed to protect the Y observerand a lead diaphragm 4 limits the pencil of rays impinging en @thesubject being observed. This subjectysuch, for example,'as a crystal 5isplacedin the path of these rays. 'A- sensitive surface 6 1s locatedbeyond theusubject. 'When'a molybvi-denum target is used, I prepare afilter fluorescent variety of the double fluoride of upon the'plate andcooperates with the trans 7 capable of becoming fluorescent under the-action of X-rays and having the filtering action of la chemical elementof an atomic weight close -to ybut 4not abovethat of molybdenum. Theelement is preferably one of slightly lower atomic weight thanlv-molybdenum in orderthat vthe K-line ofl `l0 the molybdenum radiationmayrbe absorbed,

and as Valready stated, a Vzirconium filter is.

preferable. v

Fluorescent compounds of zirconium, sucli -as zirconium fluoriderendered fluorescent by strong ignitiom are ap licable in general, butmy preferred filtering material -is the "zirconium and calcium." Thissalt may be made'up into a filter in any 'suitable way, as by ap 'lyingit with an adhesive to the surfaceo a card. When suitably placed be-ltween the source of the X-rays and the photographic surface, itoperates, to eliminate substantiall'y all of the wave-lengths exceptthose i of maximum intensity., At the same time, of the .absorbed raysis. artiallyj the ener converte into fluorescent light. his acts mittedXerays, thus reducing the time necessarly to obtain a normalphotographic image.

' t` is Vsometimes advantageous to fuse a screen 8, placed behind thephotographic surface, in combination with the inte ose'd filter justdescribed.l Such screen s ould comprise-a fluorescent materialhighly'ab-l sorptiveforlanyl X-radiation transmitted by I the filter. is wayamaxiiniim amount I ofthe energy' of the X-radiation may be utilizedjtoaffect the photographic surface. Any of vthe well known. fluorescentscreens may be placed behind the plate, such, forexample, as thosecomprisingvcalciuin tungstate.

" It will be understood however, that my inventionis'not limited to use'with such ab' `sorpt-il've screens. The filter will be placed betweenthe material under examination and the ghotogra hic film and: close tothe latter,

. than molybdenunillire used, thefil teringvand so t at the uorescentlight emitted by the i filter vmay exert the maximum ctinc effect.l

position [at any other point 1n t The use of other filters ofapproplria'te' come path-fof the -X-radiation is not precluded.- Y jWhen targets comprising elements other fluorescent material will ofcourse be suitably selectedto correspond with the element used. Thebasic idea of my invention is tlie adaptation. of the fluorescentI.material tothe par;

ticular radiation emanatingfrom the target.'

from ata'rget comprisingmolybdenum, and comprising a fluorescent.compound containing an element having an atomic weight'be'- tween S9and 96.

4. In combinatiom-a target adapted to emit X-rays, a photographicsurface, and an iiiterposed fluorescent filter comprising as its chiefconstituent an element of atomic weight just below that of the target.

fo 3. A filter for use with X-rays emanating .5. In combination, amolybdenum targetadapted to emit X-ra'ys, apliotogra hic sury face, andan interposed fluorescent fi ter com-. r prising as its chiefconstituent an elementof atomic weight between'89 and 96.

6. In combination, a molybdenum target adapted to emit .X-rays, aphotographic surface, and an' interposed fluorescent filtercontainingazirconiumcompound. 7. In cbmbin'ation, a molybdenum targetface, and an interposed fluorescent filter containing-calcium-zirconiumfluoride.'4 v

adapted to emit X-rays, a photographic sur- 8. In combination, a sourceof X-rayscom prisingfa molybdenum target, a photo-A graphic surface, a'filter comprising a fiuoresf cent zirconium compound interposed between-said source and surface, and 'a. fluorescent screen placed behindtl'ie'photographic sur.-l

face and capable 'of absorbing the major portio'n/ of the X-raystransmitted through said surface, bothl said filter and screen being sopositioned "tliat the 4light, therefrom acts -iipon the photographicsurface in cooperation `with tlie X-"ifays ltransmittedl tlirougliv thelfilter.

. lST'.y JOHN.

ich

' -In testimony'whereo'f, Iaflix my signature.

